This invention is directed to a solution for coating comprising at least one select polyarylethersulfone dissolved in a solvent comprising methylene chloride or N,N-dimethylformamide.
Polyarylethersulfones containing the following repeat unit: ##STR1## are commercially available polymers which have a wide variety of end-use applications including the ability to be cast into films. However, one of the disadvantages of the polyarylethersulfone, containing solely the above repeat unit, is the lack of low temperature boiling solvents from which cast films or adhesive solutions can be prepared. U.S. Pat. No. 3,875,103 states that the above polyarylethersulfone has instability in solvents such as methylene chloride leading to short shelf-life (column 1, lines 11 to 23). The patent states that cyclic aliphatic ketones are good solvents for the polyarylethersulfone and the resulting solutions have good shelf-life. However, methylene chloride is an inexpensive, non-toxic and widely used low boiling solvent, it would be advantageous if the polyarylethersulfone could be dissolved in it and the resulting solution retain shelf-life.
Further, the polyarylethersulfone, containing solely the above repeat unit, does not exhibit stable solutions with N,N-dimethylformamide, a water soluble solvent, which is of interest if a solution of the polyarylethersulfone is to be coagulated in water. Specifically, anisotropic membrane production would be of interest utilizing solutions of the polyarylethersulfone. As polyarylethersulfones are commercially utilized as membrane for gas separation as well as reverse osmosis applications, the solutions in N,N-dimethylformamide would be useful for such applications.
It has been found that the select polyarylethersulfone of this invention exhibit solubility in N,N-dimethylformamide.
Another commercially available polyarylethersulfone contains the following repeat unit: ##STR2## This polyarylethersulfone exhibits excellent solubility in a wide range of solvents including methylene chloride and N,N-dimethylformamide. However, a precipitate forms with methylene chloride (it takes several weeks) which must be filtered out if homogeneous films are to be prepared. The precipitate has been characterized as a cyclic dimer which crystallizes out of solution. The polyarylethersulfones of this invention have not been observed to form a precipitate with methylene chloride.
A polyarylethersulfone containing repeat units of the following structure: ##STR3## is not soluble in methylene chloride but swells and rapidly crystallizes when placed in contact with methylene chloride.
Thus, it can be seen that polymers made up of units of the following formula: ##STR4## when independently combined with the following unit: ##STR5## when connected with --O-- units do not exhibit solution stability with methylene chloride; yielding either a gelatinous material which is non-pourable or a precipitate which interfers with homogeneous film preparation.
It has now been found that the combination of two (or more) of the above units with the following unit: ##STR6## connected with --O-- units yield polymers with significantly improved solution stability in methylene chloride and N,N-dimethylformamide. Also, the resultant polyarylethersulfones of this invention have excellent solution stability after extended storage time in both methylene chloride and N,N-dimethylformamide.